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832 Discussion and Reply at its northern end. As this is the same orientation and sequence as F 1 (northeast) and F 2 (north–north-northeast) folds in the Black Range Synclinorium (Hood & Durney 2002), it implies that the earliest known folding in the Black Range Synclinorium affected also the Hervey Group in this area. (3) Fold tightness Interlimb-angle ranges from gentle to tight in both the Hervey Group (Powell et al. 1980; Raymond & Pogson 1998; Durney 2000 table 2.1) and the Black Range Synclinorium (Hood & Durney 2002 table 1). The dominant folds (north-northeast to north in the Hervey Group and north-northeast to north-northwest in the Black Range Synclinorium) are mainly open to close in both areas (Durney 2000; Hood & Durney 2002). Gentle limb dips in parts of the Parkes and Bumberry Synclines (Figure 2) may be attributable to competent granite basement near those sites (Powell et al. 1980). (4) Fold continuity The major north-northwest (F 3) syncline on the eastern side of the Black Range Synclinorium (the Taemas Synclinorium) continues north-northwestwards, via a strip of downfaulted Mountain Creek Volcanics, into similarly folded Hervey Group rocks of the Koorawatha Syncline (Brunker & Offenberg 1970; Johnston et al. 2001). These structures are part of the one major downwarp: Scheibner’s (1976) Cowra–Yass Synclinoral Zone. (5) Kinematics Kinematics of the inferred earliest (northeast–north-northeast or northeast) folds are thruststyle in both areas. For the inferred second (north-northeast–north) generation, they are thrust-style in the Hervey Group and wrench-style in the Black Range Synclinorium (Durney 2000 table 2.1; Hood & Durney 2002; figure 13). (6) Cleavage trend The dominant penetrative cleavage in mudrocks and penetrative and stripy cleavages in sandstones locally trend about north-northeast or northwest in both areas (Scott 2000 figures 2.13, 4.9; D. W. Durney unpubl. data). (7) Fissility anisotropy type Moderately to strongly cleaved mudrocks of both successions show the same, rather unusual, type of three-dimensional fissility anisotropy (Durney & Kisch 1994): steeply plunging, beddingtransverse blades, in places verging on transverse pencils (Scott 2000 figures 2.9–2.12, 4.6). Both regions have therefore experienced similar, special, deformation histories, explained as accumulated non-coaxial shortening associated with successive folding on widely divergent trends (Durney 2000; Durney & Hood 2002). Is there Middle Devonian metamorphism in the Hill End Trough? Packham refers to his 1999 interpretation of Lu et al.’s (1996b) radiometric dating in the Hill End Trough as an example of ‘Middle Devonian’ (375–364 Ma) folding and metamorphism. [Those authors considered their data to indicate ‘Carboniferous’ (363–359 Ma) folding and metamorphism.] The Hill End Goldfield, where Lu et al.’s (1996b) samples come from, shows evidence of both hydrothermal activity (bedding-parallel quartz–gold vein mineralisation) and regional metamorphism (biotite-bearing rocks with penetrative cleavage). The bedding-parallel veins, together with bedding, are folded and boudinaged and are internally deformed and recrystallised to varying degrees (Seccombe & Hicks 1989; Windh 1995). The veins thus began to form ‘before, or during the earliest stage of, the regional folding’ (Windh 1995 p. 1768). (They also show later development by slip and dilation during the folding.) Lu et al. (1996b) gave 380–370 Ma (about late Middle to early Late Devonian, depending on the time-scale used) as the age of this stage, based on 40 Ar/ 39 Ar dating of ‘early veinmuscovite’. (1) Peak temperature The 420C peak temperature that Seccombe et al. (1993 p. 424) and Packham (1999 p. 26) equated to their respective peak metamorphisms was obtained from alteration-carbonate porphyroblasts (Seccombe & Hicks 1989) associated with the mineralised bedding-parallel veins (Seccombe & Hicks 1989; Windh 1995). Therefore, the porphyroblasts definitively represent a hydrothermal event, not a metamorphic one. As they are ‘pretectonic or early syntectonic’ with respect to cleavage (Seccombe & Hicks 1989 p. 261; Windh 1995 figure 6D), they must pre-date the regional metamorphism, which should post-date (Packham 1999) or perhaps syn-date the main deformation. Thus we find no basis for assigning the early 420C event to metamorphism. (2) Peak pressure The 2.9 kb pressure estimate (290 MPa or ~11 km burial: Packham 1999) that Seccombe and Hicks (1989) and Lu and Seccombe (1993) linked to the 420C temperature was based on ‘metamorphic white mica’ from the host rocks (Lu & Seccombe 1993 p. 311) rather than on muscovite from the veins. It therefore relates to the metamorphism. As the 420C hydrothermal event was not coincident with the metamorphism, burial during the former is undefined. It could thus have been much less than 11 km, consistent with its hydrothermal nature and timing before any deposition of Upper Devonian sediment and later fold-related thickening of the sediments. Conclusions We find the terms ‘deformation’ and ‘orogeny’ insufficient to convey the nature of the Tabberabberan movement in the northeastern Lachlan Fold Belt. The two oldest Middle Devonian stratigraphical formations appear to be conformable on their respective late Emsian and earliest Eifelian limestone substrates. The Middle to early Late Devonian movements that were responsible for the Lambian unconformity in the region show predominantly very gentle (
Multiple deformation around Taemas Bridge 833 Acknowledgements Pat Conaghan and Phil Seccombe are thanked for comments that led to improvements. REFERENCES BRUNKER R. L. & OFFENBERG A. C. 1970. Goulburn 1:250 000 Geological Sheet, SI/55–12. Geological Survey of New South Wales, Sydney. CONOLLY J. R. 1965. The stratigraphy of the Hervey Group in central New South Wales. Journal and Proceedings of the Royal Society of New South Wales 98, 37–83. DURNEY D. W. 2000. Background discussion for the Taemas and Murga stops. In: Scott M. M. ed. Heterogeneous Ductile to Brittle Structures in the Eastern Lachlan Fold Belt, 15th Australian Geological Convention Field Guide FA2, pp. 15–22. Geological Society of Australia, Sydney. DURNEY D. & HOOD D. 2002. Relation of penetrative cleavage to the tectonic Y-axis in an area of non-coaxial shortening in the YZplane, Taemas, New South Wales. In: Lebit H., Luneburg C., Hudleston P. & Ramsay J. eds. 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